It is a known fact that optimum waste water treatment with activated sludge systems requires proper microbiology concentrations necessary to consume the liquid and solid organics in the waste water. Since in consuming such organic material the microorganisms reproduce and increase in number it is desirable to separate and discharge excess microorganisms.
Microbiology in the treatment system is in relatively dilute concentrations and it is desirable to concentrate the biological sludge before discharging it to other systems for further destruction. Destruction practices normally employed involve digestion of the sludge, producing a fine particulate matter which if discharged in a stream produces undesirable stream pollution.
Several factors are taken into consideration to determine the quality of treated water leaving a sewage disposal plant. These factors include B.O.D. (Biological Oxygen Demand), T.S.S. (Total Suspended Solids), D.O. (Dissolved Oxygen) and T.O.C. (Total Organic Carbon), which must be balanced with the microorganisms in order to result in the most efficient activated sludge treatment plant to result in the least contamination of streams in which the treated fluid is discharged. This is determined by making measurements on the influent and effluent of the treatment plant at various microorganism concentrations (Mixed liquor concentrations) to determine the concentration which produces optimum removal of all of the undesirable contaminates in the waste water. After such concentration point has been determined efforts must be made to control and maintain the mixed liquor concentration at or near such level.
Biological treatment systems heretofore employed, include activated sludge systems wherein excess microorganisms are removed periodically. However, the solids concentration varies over a broad range causing periods wherein treatment is less than optimum; or systems wherein the nutrients are added but mixed liquor solids are not controlled allowing the system to become overburdened with microorganisms to the extent that the biology of the system is discharged from the plant. The last named system is that typically employed in an extended aeration plant which is characterized by periodic discharges of microoganisms from the treatment plant into the effluent receiving stream.